Beads of expandable vinylaromatic polymers and process for their preparation

ABSTRACT

Beads of expandable vinylaromatic polymers which comprise: a) a matrix obtained by polymerizing 50-100% by weight of one or more vinylaromatic monomers and 0-50% by weight of at least one copolymerizable monomer; b) 1-10% by weight, calculated with respect to the polymer (a), of an expanding agent englobed in the polymeric matrix; c) 2 ppm-2% by weight, calculated with respect to the polymer (a), of an anti-lumping additive, distributed an the surface of the beads, comprising Oxides of metals of groups IB, IIB and VIIB.

The present invention relates to beads of expandable vinylaromaticpolymers and a process for their preparation.

More specifically, the present invention relates to an expandablepolystyrene in the form of beads with a reduced formation of lumps andthe process for its preparation.

Vinylaromatic polymers, and among these, polystyrene in particular, areknown products which have been adopted for a long time for preparingcompact and/or expanded articles which can be used in variousapplicative sectors, among which the most important are householdappliances, the transport and building industries, office machines, etc.A particularly interesting sector is the field of thermal insulation,where vinylaromatic polymers are essentially used in expanded form.

These expanded products are obtained by swelling, in a pre-expander,beads of expandable polymer previously impregnated with an expandingagent and molding the swollen particles inside a closed mould by meansof the contemporaneous effect of pressure and temperature.

The swelling, or pre-expansion, of the particles is generally effectedwith vapour, or another gas, maintained at a temperature slightly higherthan the glass transition temperature (Tg) of the polymer. During thisphase, the particles tend to stick to each other and to avoid thisdrawback, they are treated with additives generally known asanti-lumping additives.

The addition of metallic stearates, such as magnesium stearate, isknown, for example. This additive is extremely effective in preventingthe beads from sticking during the pre-expansion phase but results inpoor adhesion in the moulding phase during the preparation of theend-products.

U.S. Pat. No. 3,520,833 describes the use of lecithin, added with theexpanding agent during the impregnation of the particles, asanti-lumping additive. Unfortunately, lecithin gives the end-products anunacceptable odour making their use problematical.

U.S. Pat. No. 3,444,104 describes the use of calcium and siliconaluminates. The lumps, in this case, are eliminated and the adhesionremains acceptable. The density of the beads in expansion, however, ispoor.

U.S. Pat. No. 3,462,293 describes the use of a polymeric latex, forexample polyethylene or SAN copolymer, deposited on the particles. Thesubsequent drying treatment of the latex with air, however, requireslong times which make the preparation process quite unsuitable forindustrial development.

Finally, European patent 449,065 describes the use of oxides such assilica, alumina and titanium oxide. Although these products have a goodcapacity against the formation of lumps, they have the disadvantage, asspecified above, of reducing the adhesion of the expanded beads duringthe moulding phase.

There are evident disadvantages associated with these systems foravoiding the formation of lumps as they either jeopardize the adhesionof the expanded beads, or the expansion of the beads themselves or theyrequire processes, for example drying, which are too elaborate for beingconveniently used in the industrial field.

The Applicant has now found a simple system for eliminating theformation of agglomerates of swollen vinylaromatic polymer beads withoutjeopardizing the subsequent adhesion and/or expandability.

An object of the present invention therefore relates to beads ofexpandable vinylaromatic polymers which comprise:

-   a) a matrix obtained by polymerizing 50-100% by weight of one or    more vinylaromatic monomers and 0-50% by weight of at least one    copolymerizable monomer;-   b) 1-10% by weight, calculated with respect to the polymer (a), of    an expanding agent englobed in the polymeric matrix;-   c) 2 ppm-2% by weight, calculated with respect to the polymer (a),    of an anti-lumping additive, distributed on the surface of the    beads, selected from oxides of metals of groups IB and VIIIB or from    mixtures consisting of oxides of metals of groups IB, IIB and VIIIB    and esters of C₈-C₂₅ fatty acids with the same metals.

A further object of the present invention relates to a process for thepreparation of expandable vinylaromatic polymer beads which comprises:

-   -   polymerizing 50-100% by weight of one or more vinylaromatic        monomers and 0-50% by weight of at least one copolymerizable        monomer;    -   englobing an expanding agent in the polymeric matrix; and    -   distributing on the surface of the beads obtained 2 ppm-2% by        weight, calculated with respect to the polymer, of an        anti-lumping additive selected from oxides of metals of groups        IB and VIIIB or from mixtures consisting of oxides of metals of        groups IB, IIB and VIIIB and esters of C₈-C₂₅ fatty acids with        the same metals.

According to the present invention, the vinylaromatic polymers can beobtained with a polymerization process in aqueous suspension orcontinuous mass. In particular, the polymerization in suspension of thevinylaromatic monomer is carried out under conditions and in thepresence of additives well known in the art. In addition to thesuspending agent, selected from insoluble inorganic salts, for examplecalcium or magnesium such as tricalcium phosphate or magnesiumphosphate, or from organic suspending agents such aspolyvinylpyrrolidone, the polymerization is carried out in the presenceof an initiating system or an expanding system.

The initiating system generally comprises two peroxides, one with ahalving time of an hour at 85-95° C. and the other with a halving timeof an hour at 110-120° C. Examples of these initiators arebenzoylperoxide and terbutylperbenzoate.

Any expanding agent capable of being englobed in the polymeric matrixcan be used in a combination with the vinylaromatic polymers object ofthe present invention. In general, liquid substances are used, with aboiling point ranging from 10 to 100° C., preferably from 20 to 80° C.Typical examples are aliphatic hydrocarbons, freon, carbon dioxide,water, etc.

The expanding agent can be added to the polymer during thepolymerization phase in suspension, or, alternatively, by impregnationof the beads produced at the end of the polymerization or by injectioninto the molten polymer according to the continuous mass preparationprocess. At the end of the addition, a polymer is obtained in the formof beads, which can be transformed to produce expanded articles with adensity ranging from 5 to 50 g/l, preferably from 8 to 25 g/l, with anexcellent thermal insulation capacity. In order to favour the retentionof the expanding agent in the polymeric matrix, additives capable offorming bonds both of the weak type (for example hydrogen bridges) orstrong type (for example acid-base adducts), can be used with theexpanding agent. Examples of these additives are methyl alcohol,isopropyl alcohol, dioctylphthalate, dimethylcarbonate, compoundscontaining an amine group, etc. These additives are generally addedduring the polymerization and/or englobed in the polymer together withthe expanding agent.

The expanding agents are preferably added during the polymerizationphase and are selected from aliphatic or cycloaliphatic hydrocarbonscontaining from 3 to 6 carbon atoms, such as n-pentane, iso-pentane,cyclopentane or their mixtures; halogenated derivatives of aliphatichydrocarbons containing from 1 to 3 carbon atoms such as, for example,dichlorodifluoromethane, 1,2,2-trifluoroethane, 1,1,2-trifluoroethane;carbon dioxide and water.

The term “vinylaromatic monomer”, as used in the present description andclaims, essentially refers to a product which corresponds to thefollowing general formula:

wherein n is zero or an integer ranging from 1 to 5, R is a hydrogenatom or a methyl and Y is a halogen, such as chlorine or bromine, or analkyl or alkoxyl radical having from 1 to 4 carbon atoms.

Examples of vinylaromatic monomers having the general formula definedabove are: styrene, α-methylstyrene, methylstyrene, ethylstyrene,butylstyrene, dimethylstyrene, mono-, di-, tri-, tetra- andpenta-chlorostyrene, bromostyrene, methoxy-styrene, acetoxy-styrene,etc. Styrene and α-methylstyrene are the preferred vinylaromaticmonomers.

The vinylaromatic monomers having general formula (I) can be used aloneor in a mixture of up to 50% by weight with other copolymerizablemonomers. Examples of these monomers are (meth)acrylic acid, C₁-C₄ alkylesters of (meth)acrylic acid, such as methyl acrylate,methylmethacrylate, ethyl acrylate, ethylmethacrylate, isopropylacrylate, butyl acrylate, amides and nitriles of (meth)acrylic acid suchas acrylamide, methacrylamide, acrylonitrile, methacrylonitrile,butadiene, ethylene, divinylbenzene, maleic anhydride, etc. Preferredcopolymerizable monomers are acrylonitrile and methylmethacrylate.

The vinylaromatic polymer or copolymer which is obtained has a molecularweight Mw ranging from 50,000 to 250,000, preferably from 70,000 to200,000. In general, greater details on a process for the preparation ofexpandable vinylaromatic polymers in aqueous solution or, moregenerally, on polymerization in suspension, can be found in Journal ofMacromolecular Science, Review in Macromolecular Chemistry and Physicsc31 (263) 215-299 (1991) or in international patent application WO98/51734.

Conventional additives, generally used with commercial materials, suchas pigments, stabilizers, flame-retardant agents, antistatic agents,detaching agents, shock-resistant agents, etc., can be added to theexpandable vinylaromatic polymers obtained with the process, object ofthe present invention. In particular, it is preferable to add mineralfillers of athermanous materials during the polymerization, such asgraphite or refracting agents such as titanium dioxide, in a quantityranging from 0.05 to 25% by weight, calculated with respect to theresulting polymer.

At the end of the polymerization, substantially spherical polymer beadsare obtained, with an average diameter ranging from 0.2 to 2 mm, insidewhich the expanding agent is homogeneously dispersed.

To improve the stability of the suspension, it is possible to increasethe viscosity of the reagent solution by dissolving the vinylaromaticpolymer therein, in a concentration ranging from 1 to 30% by weight,preferably from 5 to 20%, calculated with respect to the monomer alone.The solution can be obtained either by dissolving a preformed polymer(for example fresh polymer or the waste products of previouspolymerizations and/or expansions) in the reagent mixture or bypre-polymerizing the monomer, or mixture of monomers, in mass, in orderto obtain the above concentrations, and then continuing thepolymerization in aqueous suspension in the presence of the remainingadditives.

At the end of the polymerization, the beads are discharged from thepolymerization reactor, washed and dried.

The beads produced are subjected to pre-treatment generally applied totraditional materials and which essentially consists in:

-   1. coating the beads with a liquid antistatic agent such as amines,    tertiary alkylamines, ethylene oxide-propylene oxide copolymers,    etc. The purpose of this agent is to facilitate both the adhesion of    the coating and also the screening of the beads prepared in    suspension;-   2. applying the coating to the above beads, said coating essentially    consisting of a mixture of mono-, di- and tri-esters of glycerin (or    other alcohols) with fatty acids; and-   3. distributing on the surface of the beads, the anti-lumping    additive preferably selected from powders of α-ides of iron (Fe₂O₃),    copper (CuO) and zinc (ZnO), optionally mixed with the corresponding    ester of fatty acids such as stearic acid, palmitic acid, myristic    acid, etc. The anti-lumping additive is generally used in the form    of powders with an average particle-size ranging from 0.1 to 50 μm.

Some illustrative but non-limiting examples are provided for a betterunderstanding of the present invention and for its embodiment.

COMPARATIVE EXAMPLE 1

100 parts of styrene monomer, 0.30 parts of benzoyl peroxide, 0.15 partsof terbutylperbenzoate, 100 parts of demineralized water, 0.2 parts oftricalcium phosphate and 20 ppm of sodium dodecylbenzenesulfonate, werecharged into a 2 litre reactor equipped with a stirrer.

The mixture was then heated to 90° C. for 4 hours and to 125° C. for afurther 6 hours.

The expanding agent (7 parts of n-pentane) was added during the risefrom 90 to 125° C.

At the end of the polymerization, the beads were centrifuged, washedwith water and dried with air at room temperature.

0.02% of diethanol alkyl amine was added to the beads of expandablepolymer thus produced, which were then sieved, separating the fractionwith a diameter ranging from 0.4 to 0.6 mm.

0.25% of glycerylmonostearate and 0.1% of magnesium stearate were thenadded to the fraction.

The product is pre-expanded the following day with vapour at atemperature of 100° C. at a density of 20 g/l, evaluating the quantityof clots, which proved to be practically zero.

The expanded beads are then aged for a day and used the following dayfor the moulding of boxes (thickness 20 mm) at a pressure of 0.5 bars.The adhesion of the beads proved to be 5%.

COMPARATIVE EXAMPLE 2

The beads polymerized according to Comparative example 1 were lubricatedwith 0.02% of diethanol alkyl amine and sieved, separating the fractionwith a diameter ranging from 0.4 to 0.6 mm.

0.25% of glycerylmonostearate and 0.1% of zinc stearate were then addedto the fraction.

The product is pre-expanded the following day with vapour at atemperature of 100° C. at a density of 20 g/l, evaluating the quantityof lumps, which proved to be equal to 3%.

The expanded beads are then aged for a day and used the following dayfor the moulding of boxes (thickness 20 mm) at a pressure of 0.5 bars.The adhesion of the beads proved to be 40%.

COMPARATIVE EXAMPLE 3

The beads polymerized according to Comparative example 1 were lubricatedwith 0.02% of diethanol alkyl amine and sieved, separating the fractionwith a diameter ranging from 0.4 to 0.6 mm.

0.25% of glycerylmonostearate and 0.1% of silica Sipernat D 17 (diameterof 10 μm) of Degussa, were then added to the fraction.

The product is pre-expanded the following day with vapour at atemperature of 100° C. at a density of 20 g/l, evaluating the quantityof lumps, which proved to be absent.

The expanded beads are then aged for a day and used the following dayfor the moulding of boxes (thickness 20 mm) at a pressure of 0.5 bars.The adhesion of the beads proved to be 5%.

COMPARATIVE EXAMPLE 4

The beads polymerized according to Comparative example 1 were lubricatedwith 0.02% of diethanol alkyl amine and sieved, separating the fractionwith a diameter ranging from 0.4 to 0.6 mm.

0.25% of glycerylmonostearate and 0.1% of alumina, with a diameter <10μm, commercialized by Aldrich, were then added to the fraction.

The product is pre-expanded the following day with vapour at atemperature of 100° C. at a density of 20 g/l, evaluating the quantityof lumps, which proved to be absent.

The expanded beads are then aged for a day and used the following dayfor the moulding of boxes (thickness 20 mm) at a pressure of 0.5 bars.The adhesion of the beads proved to be 5%.

EXAMPLE 1

Comparative example 2 is repeated, reducing the zinc stearate to 0.05%and adding 0.05% of zinc oxide. There were no lumps and the adhesion wasexcellent, equal to 70%.

EXAMPLE 2

Comparative example 1 is repeated, substituting the magnesium stearatewith 5 ppm of ferric oxide in powder form, produced by Aldrich, with anaverage particle diameter of about 1 μm. There were no lumps and theadhesion was excellent, equal to 80%.

EXAMPLE 3

Comparative example 1 is repeated, substituting the magnesium stearatewith 5 ppm of copper oxide (CuO) in powder form, produced by Aldrich,with an average particle diameter <5 μm. There were no lumps and theadhesion was excellent, equal to 70%.

1. Beads of expandable vinylaromatic polymers comprising: a) a matrixobtained by polymerizing 50-100% by weight of one or more vinylaromaticmonomers and 0-50% by weight of at least one copolymerizable monomer; b)1-10% by weight, calculated with respect to the polymer (a), of anexpanding agent englobed in the polymeric matrix, c) 2 ppm-2% by weight,calculated with respect to the polymer (a), of an anti-lumping additive,distributed on the surface of the beads, selected from oxides of metalsof groups IB and VIIIB or from mixtures consisting of oxides of metalsof groups IB, IIB and VIIIB and esters of C₈-C₂₅ fatty acids with thesame metals.
 2. The beads of expandable vinylaromatic polymers accordingto claim 1, having an average molecular weight Mw ranging from 50,000 to250,000.
 3. The beads of expandable vinylaromatic polymers according toclaim 1, wherein the beads are substantially spherical with an averagediameter ranging from 0.2 to 2 mm.
 4. The beads of expandablevinylaromatic polymers according to claim 1, wherein the beads comprisefillers of athermanous materials in quantity ranging from 0.05 to 25% byweight.
 5. A process for the preparation of beads of expandablevinylaromatic polymers comprising: polymerizing 50-100% by weight of oneor more vinylaromatic monomers and 0-50% by weight of at least oneco-polymerizable monomer; englobing an expanding agent in the polymericmatrix; and distributing on the surface of the beads obtained 2 ppm-2%by weight, calculated with respect to the polymer, of an anti-lumpingadditive selected from oxides of metals of groups IB and VIIIB or frommixtures consisting of oxides of metals of groups IB, IIB and VIIIB andesters of C₈-C₂₅ fatty acids with the same metals.
 6. The processaccording to claim 5, wherein the polymerization is carried out inaqueous suspension or in continuous mass.
 7. The process according toclaim 5, wherein the polymerization is carried out in suspension in thepresence of a suspending agent, an initiating system and an expandingsystem.
 8. The process according to claim 7, wherein the expandingsystem consists of liquid substances with a boiling point ranging from10 to 100° C.
 9. The process according to any of the previous claimsclaim 5, further comprising:
 1. coating the beads with a liquidantistatic agent such as amines, tertiary alkylamines, ethyleneoxide-propylene oxide copolymers;
 2. applying the coating to the abovebeads, said coating essentially consisting essentially of a mixture ofmono-, di- and tri-esters of glycerin (or other alcohols) with fattyacids; and
 3. distributing on the surface of the beads, the anti-lumpingadditive preferably selected from the group consisting of powders ofoxides of iron (Fe₂O₃), copper (CuO) and zinc (ZnO), optionally mixedwith the corresponding ester of fatty acids.
 10. The process accordingto claim 5, wherein the anti-lumping additive is a powder with anaverage particle-size ranging from 0.1 to 5 0 μm.